1. Field of the Invention
The present invention relates to electroluminescent structures and, more particularly, to an electroluminescent device and method having increased brightness and resolution for electroluminescent displays.
2. Description of the Related Art
Electroluminescent (EL) displays produce light when an alternating current (AC) voltage is applied across a phosphor film sandwiched between a pair of electrodes. If an organic material is used, a similar structure is used however excitation of the organic material is performed in a different manner, for example using DC current. Referring to FIG. 1, electroluminescent light originates from metal activator atoms that are introduced into a phosphor film 12 and excited by energetic electrons as they move across the semi-conducting phosphor film 12. Since the phosphors employed have large band gaps, visible radiation produced (indicated by arrows) passes through film 12 without absorption and out of the stack through a transparent electrode 14.
The typical EL film stack contains two dielectric layers 16 and 18, one at each electrode interface, i.e. one for transparent electrode 14 and one for electrodes 20. These dielectric layers limit the current through the structure and prevent a catastrophic breakdown should a phosphor imperfection produce a conductive path through film 12. Dielectric layers 16 and 18 also store charge, increase the internal electric field and reduce the effective turn-on voltage of the phosphor. Thin 500 to 1000 A films with high dielectric constants are often used to enhance the effect and increase the luminous efficiency of EL displays.
Compact high-resolution displays have been produced with on-chip scanning and pixel control circuitry. In these "active matrix" displays, the necessary dielectric, phosphor and transparent electrode layers are deposited and defined as a single rectangle over the entire pixel array. Referring again to FIG. 1, individual pixel electrodes 20 are controlled by switching a transistor 22 which blocks the AC phosphor excitation voltage 24 when "off" and allows passage current through the phosphor when "on". Pixel electrodes 20 are positioned directly over the controlling transistors, to maximize resolution.
While this architecture addresses the information content and size requirements of small displays, the structure limits the brightness and resolution achievable. Pixel electrodes translate the underlying topology of the active matrix array and present an irregular surface that does not efficiently reflect phosphor radiation toward a viewer. Light emitted from one pixel can migrate from the electrode to neighboring pixels through lateral emission and internal reflection in the phosphor film stack to degrade resolution and color spectral purity as shown in FIG. 1. Pixel electrode structures that maximize the phosphor emission and transmission efficiency and minimize lateral light diffusion are needed to satisfy the requirements for high brightness, color and high-resolution display products.